1. Field of the Invention
The present invention relates to a planar motor initialization method, a planar motor, a lithographic apparatus, and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a lithographic patterning device, which is alternatively referred to as a “mask” or “reticle”, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (i.e., resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, while in so-called scanners, each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The substrate and the patterning device are being moved and transferred into, and inside and out of, the lithographic apparatus. A planar motor may be employed for one or more transfers and moves of the substrate and/or the patterning device.
A planar motor comprises a magnet plate and a coil assembly. The magnet plate is a two dimensional array of magnets having alternating magnet poles. The coil assembly comprises a plurality of coils, preferably three coils. These coils are elongated coils, the separate coils may be positioned parallel to each other. The coil assembly is positioned over the alternating magnet poles of the magnet plate.
The planar motor may be provided with a control system for providing a current to the coils. The current is an alternating current, preferably a two-phase or three-phase alternating current. When current flows through the coils positioned in a magnetic field, a force is generated which force is exerted on the coil assembly relative to the magnet plate. A direction of the force depends on the direction of the current through the coils and on the relative position of the coils with respect to the magnetic field.
Hereinafter, it is presumed that the magnet plate is mounted and attached to a frame, for example, a frame of the lithographic apparatus and thus functions as a stator of the planar motor, and the coil assembly may move relative to the magnet plate. This, however, is not essential and it may be that the coil assembly functions as a stator and the magnet plate may move, or both may move relative to each other.
Assuming a magnet plate extending in the horizontal plane, during operation, a vertical force may be generated in order to generate a gap between the coil assembly and the magnet plate. Thus, the coil assembly may levitate above the magnet plate. Further, a horizontal force may be generated in order to move the coil assembly horizontally relative to the magnet plate. If both forces are generated simultaneously, the coil assembly may be controlled to move in a plane parallel to the magnet plate with only little friction.
In rest, i.e. not in operation, there may be no vertical force and the coil assembly is positioned on the magnet plate. To start the planar motor, the motor needs to be initialized. Since the direction of a generated force depends on the direction of the current and on the relative position of the coil assembly and the magnetic field, this relative position should be determined in order to control the coil assembly correctly.
It is known to provide the coil assembly with hall sensors to determine the relative position of the magnetic field at the coil assembly directly. Further, it is known to provide a position determination system for determining the position of the coil assembly with respect to the magnet plate directly. Both known methods disadvantageously require additional electronic elements, which are sensitive to malfunctioning, require additional circuitry and add additional weight.